1. Field of Invention
This invention relates generally to generation of steam and more particularly to generation of steam from a feedwater inlet stream including impurities.
2. Description of Related Art
Steam is generated for use in many industrial processes. The generated steam may be used to perform mechanical work, for heating and temporary energy storage, and for generating electricity. In hydrocarbon recovery operations steam is additionally used for extracting heavy oil through cyclic steam stimulation, steam flooding, or steam-assisted gravity drainage (SAGD), for example. The cost of steam generation and the associated generation of emissions is a major consideration in assessing economic potential of hydrocarbon recovery operations.
In SAGD operations steam is generated from a feedwater stream which may include groundwater, surface water, and fresh water. Water produced from the SAGD well is commonly treated and at least a portion is recycled for use in the steam generation. Heat for operating steam generators may be provided by natural gas, synthetic gas generated by gasification of heavy fractions of produced bitumen, coal, or a nuclear reactor, for example.
SAGD operations generally require injection of dry steam having a steam quality of substantially 100%, because any liquid content injected into the steam chamber essentially drains to the production borehole and requires removal and recycling. This adds to the water recycling costs without contributing to heat delivery or hydrocarbon recovery within the well. The steam quality is expressed as a percentage of the mass of the stream that is in the vapor state, with dry steam having a quality of 100%. Steam generation generally follows a two-stage process in which steam is generated at a quality of less than 80% followed by a subsequent processing through a steam separator that produces a stream of substantially dry 100% quality vapor and a liquid stream, which is typically treated and recycled.
US patent application 2011/0017449 by Berruti discloses a steam generator having a steam-generating circuit having a heating segment defining a heating portion of the steam generator. The steam-generating circuit includes a pipe that receives feedwater at an inlet end and is subjected to heat to convert the feedwater into steam and water. The pipe has a bore at least partially defined by an inner surface having ribs defining a helical flow passage, which guides and imparts a swirling motion to the water to control concentrations of the impurities in the water. Berruti discloses that rifled pipes offer the ability to operate at higher steam quality without significantly increasing the surface impurity concentration level, thus reducing the likelihood of over-saturating the impurity components in which case scale may form. Berruti also discloses that wetted wall conditions result in more efficient heat transfer and the heat transfer coefficient of the steam flow is considerably higher in wetted wall versus dry conditions.